MIPS: bcm63xx: nvram: avoid inefficient use of crc32_le_combine()

MIPS: bcm63xx: nvram: avoid inefficient use of crc32_le_combine()

bcm963xx_nvram_checksum() was using crc32_le_combine() to update a CRC
with four zero bytes.  However, this is about 5x slower than just
CRC'ing four zero bytes in the normal way.  Just do that instead.

(We could instead make crc32_le_combine() faster on short lengths.  But
all its callers do just fine without it, so I'd like to just remove it.)

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>